1. Technical Field
The present invention relates to a rotor for an eddy current type retarder.
2. Background Art
A large vehicle such as a bus or a truck requires a large brake force to ensure safe and stable braking, particularly, for example, during long down-slope driving. An eddy current type retarder has been employed to realize such large braking force by producing a secondary braking force in addition to a primary braking force produced by a foot brake and another secondary braking force produced by an exhaust brake.
In an eddy current type retarder, a shaft and a rotor are rotated together in a magnetic field formed by magnets such that eddy current generated by the rotor produces the brake force. The rotor usually has fins on the outer surface thereof to release heat produced by the eddy current.
Meantime, it is required for the eddy current type retarder to generate a large eddy current and to effectively release the heat generated by the eddy current. In addition, with respect to material, the retarder has to be ferromagnetic, rigid against heat, electro-conductive and heat-conductive.
Heretofore, it has been proposed to manufacture the rotor from low-carbon steel. However, it is difficult to make the rotor from low-carbon steel since the shape of the fins is very complicated. Specifically, grinding or cutting of fins of complicated shape is necessary, or casting cast iron, whose castability is poor, is necessary. Either approach raises the manufacturing cost of the rotor. In addition, the rotor of low-carbon steel easily gathers rust since the rotor faces and is spaced from the magnet of the retarder. The rust lowers the braking efficiency.